It is well known in the art that polyolefins are inherently inert and that a chemical change is required to obtain an active surface which is receptive to inks and adhesives. The surface of the object must be oxidized in order to provide a chemical bond. The oxidation of shaped objects can be accomplished by a variety of methods, including gas flame treatment, electronic spark discharge with opposing electrodes, chemical oxidation with various chemical compounds, and UV irradiation treatment. However, there are problems associated with each of these methods.
The gas flame treatment method is accomplished by exposing the surface of the shaped object to a gas flame. However, flame treatment may result in unsatisfactory surface treatment, since the surface is liable to be distorted, burned or melted by the flame. Another disadvantage arises in the expense associated with providing compressed air for combustion of the flame. Additionally, the use of gas flames causes a possible fire hazard. Flame treatment also generates heat and cannot be instantly turned on and off.
Electronic spark discharge treatment is carried out by passing the object to be treated through a tunnel within which a pair of opposing electrode plates are disposed. Ionized gases, also known as atmospheric or nonvaccuum plasmas or Coronas are created by the electricity passing through the air. A high AC voltage is impressed between the electrodes to effect spark discharge. An electric current flows through the surface of the object which is parallel with the direction of the spark discharges. This system is mainly used in treating films and flat objects since this method only treats the surfaces which are parallel to the direction of the discharge. An example of this method is shown in U.S. Pat. No. 3,484,363 to Williams incorporated herein by reference. Williams generally describes an electrical discharge treating apparatus designed to provide a plurality of uniform spark discharges, having a pair of electrodes spaced apart from each other within a tunnel through which plastic objects pass to be treated. A horizontally disposed pair of electrodes generate vertical sparks, and a pair of vertically disposed electrodes generate horizontal sparks to treat both the vertical and horizontal surfaces of the passing object. The disadvantage of the Williams apparatus is that the distance between the electrodes determines the maximum size of object which can be treated, and the larger the gap between the electrodes, the greater the electrical stresses. Another disadvantage is the fact that the apparatus requires a tunnel chamber and gaseous mixtures for suitable treatment.
Chemical oxidation can be accomplished by exposing the surface of the object to a low temperature plasma so that the surface undergoes a chemical change by the action of the oxygen and ultra violet rays, however this method requires very expensive apparatus and a sealed treatment chamber. Another treatment method is to immerse the shaped body into a chromic acid mixture, but immersion results in high costs for the treatment of the waste produced. Although the entire 360.degree. surface area of the shaped object can be treated by immersion methods, these methods are costly and relatively slow.
UV irradiation requires contacting the surface with a UV ray-absorbing liquid by immersion or any suitable coating method such as spray coating, and then subjecting the object to UV irradiation treatment. Examples of UV irradiation treatment of shaped objects are generally illustrated in U.S. Pat. No. 5,077,082 to Katoh, et al. incorporated herein by reference. Katoh, et al. generally describes a method of treating the surface of a shaped body formed of polypropylene resin by first contacting the surface with a UV ray-absorbing liquid by immersion or any suitable coating method such as spray coating, and then subjecting the object to UV irradiation treatment using, for instance, a low or medium pressure mercury lamp. One major disadvantage of the system described in Katoh, et al. is that UV irradiation treatment requires a relatively long period of time in order to obtain the desired results. The same disadvantages as arise in immersion of the shaped object discussed above, such as expensive apparatus and the necessity of a sealed treatment chamber, also make this method undesirable.
Certain treatment methods are combined to provide greater results in the treatment of polymer surfaces. U.S. Pat. No. 5,059,447 to Katoh, et al. generally describes a method of treating the surface of a shaped body formed of a synthetic organic polymer by spark discharge treatment which is preceded and/or followed by a treatment with a UV ray-absorbing liquid and/or UV irradiation treatment. Although the combination method decreases the time required for the UV irradiation step, the entire treatment time is still relatively slow and unsuitable for high speed production lines.
One major disadvantage associated with treatment of shaped objects which require 360.degree. of surface area to be treated, such as bottles, cylindrical containers or the like, is the difficulty in treating the entire 360.degree. surface area in one step. Since manufacturers usually desire to label or print on all sides of the container, an economical and inexpensive method for treating the entire 360.degree. surface area in one step is needed. The systems in use today effectively treat only the side of the shaped object which is parallel to the treatment means. Thus when treatment means such as flame, electronic discharge, UV lamp, or sprayer are used, the entire 360.degree. surface area of the object is not fully treated. In order to obtain treatment of the entire 360.degree. surface area of a shaped object, the object must be passed by the treatment means multiple times or there must be multiple treatment means set up along all sides of the object. Multiple treatments are unsuitable for industry use, due to reduction in the speed of production, excess time and energy use, and increased cost. Although chemical coating by complete immersion will treat the entire 360.degree. surface area of the object, this method is not suitable for production line and is costly and inefficient.
One possible solution to these problems is to provide a system wherein the entire 360.degree. surface area of the object is fully treated in one step.
Another possible solution to these problems is to provide a system wherein the treatment method is economical and requires a relatively short period of time in order to obtain the desired results, and can be adapted for use with existing high speed production lines.
Another possible solution to these problems is to provide a system wherein the treatment device which generates only a small amount of heat, and does not increase fire hazards.
Thus, there has been a need in the art for a system that provides treatment of the entire 360.degree. surface area of the object in one step.
There is an additional need in the art for a system which is suitable and adaptable for industry use in high speed production lines.
There is an additional need in the art for a system which the treatment method requires a relatively short period of time in order to obtain the desired results.
There is an additional need in the art for a system in which does not require very expensive apparatus.
There is an additional need in the art for a system which the treatment device does not generate extreme amounts of heat, or increase fire hazards.